The effectiveness of embedded-lens retroreflective sheeting, as first described in U.S. Pat. No. 2,407,680, depends on the existence of rather precise optical relationships within the sheeting.
The sheeting comprises a monolayer of transparent microspheres, a transparent polymeric cover layer covering the front surfaces of the microspheres, a transparent polymeric spacing layer covering the back surfaces of the microspheres, and a specularly reflective layer coated on the back surface of the spacing layer. Retroreflection depends on there being an appropriate relationship of index of refraction between the transparent cover layer, spacing layer, and transparent microspheres, and on the spacing layer having an appropriate thickness such that light rays passed through the microspheres are focused onto the specularly reflective coating on the back surface of the spacing layer, where the light rays are reflected and returned back toward the original source of the light.
The need for precise optical relationships is perhaps a primary reason why, so far as known, there has never been an elastomeric version of embedded-lens retroreflective sheeting. The repeated stretching of the elastomeric spacing layer would be expected to disturb the needed optical relationship between the microspheres and specularly reflective coating on the back surface of the microspheres. For example, when a flat sheet of a transparent elastomeric material is coated with a specularly reflective coating and then repeatedly stretched and relaxed, reflection from the specularly reflective coating rapidly declines to a small fraction of its original brightness. Apparently the stretching rapidly cracks and disrupts the specularly reflective coating such that the layer loses its reflectivity.
There have been forms of elastomeric reflective sheeting before, for example, the sheet material taught in U.S. Pats. No. 3,382,908 and 3,449,201. But that sheeting was an exposed-lens form of sheeting in which a specularly reflective coating is placed directly on the back surface of the microspheres, and no polymeric material is interposed in the path of light incident on the sheeting and the specularly reflective coating. Accordingly the optical system, i.e., the lens structure and specularly reflective layer through which light passes, is never stretched.
U.S. Pat. No. 3,551,025 discloses a highly flexible embedded-lens retroreflective sheeting having an elastomeric transparent topcoat and binder layer, but the sheeting again avoids the use of a spacing layer between the microspheres and specularly reflective layer, meaning that there is no stretching of the specularly reflective layer, very high-index microspheres, e.g., above 2.4, and low-index polymers, e.g. 1.39, were used, and the specularly reflective layer was applied directly to the microspheres.
U.S. Pat. No. 4,418,110 teaches a non-elastomeric retroreflective sheeting which is intentionally stretched to rupture the specularly reflective laYer and make the sheeting permeable to vapor. The sheeting is stretched at elevated temperature and is stretched only once, and under those conditions it is found that retroreflection is retained.